In an oxide superconductor, because that critical temperature (Tc) exceeds the liquid nitrogen temperature, the applications for such as the superconducting magnet, the superconducting electric power cable, the electric power equipment and the device are expected, and many study results are informed.
In order to apply the oxide superconductor to the above-mentioned field, it is necessary to produce a long wire which has a high critical current density (Jc) and a high critical current value (Ic). On the other hand, in order to obtain a long wire, it is necessary to form the oxide superconductor onto a metallic substrate from viewpoint of strength and flexibility. Also, in order to enable use of the oxide superconductor at a practical level which is equivalent to the metallic superconductor such as Nb3Sn or Nb3Al, the Ic value of about 500 A/cm (at 77K, in self-field) is required.
Besides, in the oxide superconductor, because the superconducting characteristic changes by that crystal orientation, it is necessary to improve the in-plane orientation for improving Jc, and it is necessary to form the oxide superconductor onto the tape-shaped substrate. For this reason, the film fabrication process that the oxide superconductor performs the epitaxial growth onto the high in-plane orientation substrate is adopted.
In this case, for improving Jc, it is necessary that the c axis of the superconductor is oriented perpendicular to the surface of the substrate, the in-plane orientation of the a axis (or the b axis) is conducted parallel to the surface of the substrate, and the quantum connectivity of the superconducting state is maintained well. For this reason, the improvements of the in-plane orientation and the azimuth of the crystal of the superconducting layer are conducted by forming the intermediate layer which improves the in-plane orientation and the azimuth onto the metallic substrate whose in-plane orientation is high, and by using the crystal lattice of this intermediate layer as the template. Besides, for improving Ic, it is necessary to thicken the film thickness of the oxide superconductor which is formed onto the substrate.
As the process for producing the tape-shaped RE-type oxide superconductor, that is, as the process for producing REBa2Cu3Oz type oxide superconductor (here, RE means at least more than one kind of element selected from Y, Nd, Sm, Gd, Eu, Yb, Pt or Ho. Hereinafter called RE-type (123) superconductor.), MOD process (Metal Organic Deposition Processes) is known.
This MOD process makes metal organic acid salt decompose thermally. And after coating a solution that an organic compound which includes a metallic component which composes the superconductor solves uniformly onto the substrate, by heating this and then by making this decompose, the thin film is formed onto the substrate. This MOD process is non-vacuum process. Therefore, because the crystal growth of the film is possible at low cost and high speed, and because high Jc is obtained, there is an advantage that this process is suitable to produce the long tape-shaped oxide superconducting wire.
In MOD process, when the metal organic acid salt as starting material is decomposed thermally, the carbonate of alkali earth metal (Ba, etc.) is usually formed. However, in the formation of the oxide superconductor by a solid-phase reaction through this carbonate, the high temperature heat treatment more than 800 degrees C. is required. In addition, when the thick film is formed, because the nucleation for crystal growth occurs also from, parts other than the interface of the substrate, it is difficult to control the crystal growth rate. Consequently, there is a problem that it is difficult to obtain the superconducting film which has superior in-plane orientation, that is, the high Jc.
For solving the above-described problem in MOD process, as the process for producing the RE-type (123) superconductor without going through the carbonate, by using organic acid salt (for example, TFA salt: trifluoroacetate) including fluorine as the starting material, recently, the process for obtaining the superconductor through decomposition of fluoride by conducting the heat treatment under control of a water vapor partial pressure in a water vapor atmosphere is conducted vigorously.
In MOD process which uses this TFA salt as the starting material, by reaction between amorphous precursor including fluorine which is obtained after a preliminary calcination of the coating film and the water vapor, the epitaxial growth of the superconductor is conducted from the interface of the substrate by forming a liquid phase which is caused by HF in the interface that the superconducting film grows while HF gas occurs. In this case, because the decomposition speed of the fluoride can be controlled by the water vapor partial pressure during the heat treatment, the crystal growth rate of the superconductor can be controlled, consequently, the superconducting film which has the superior in-plane orientation can be produced. Besides, in this process, the epitaxial growth of the RE-type (123) superconductor can be conducted front the upper surface of the substrate at the comparatively low temperature.
Heretofore, for enabling the formation of the thick film and the high speed preliminary calcination process, the mass generation of HF gas in the preliminary calcination process is restrained by using the solution that TFA salt of Y and Ba and naphthenate of Cu are mixed in the organic solvent with the molar ratio of Y:Ba:Cu=1:2:3 as the starting materials.
As described above, when the tape-shaped oxide superconductor is produced by the MOD process, the formation of the thick film to improve the Ic value is essential for practical application. For accomplishing the formation of the thick film by MOD process when the starting material is TFA salt, there are thoughts that the viscosity of the raw material solution including TFA salt is increased and the coating film is thickened. However, when the thickness of the coating film per once becomes thick, because the quantity of gas generation of HF and CO2 by decomposition in heat treatment increases, the phenomenon that the coating film is scattered in preliminary calcination occurs. Consequently, it is difficult to produce the thick film of the tape-shaped oxide superconductor which has the high characteristic.
For producing the thick film of the superconductor, the process for thickening the preliminary calcination film is considered by repeating the operations of the coating of the material and the preliminary calcination. However, in the above-mentioned heat treatment method by the preliminary calcination of conventional technology, because the rate of temperature rise which affects the decomposition speed of the metal organic acid salt in the heat treatment for preliminary calcination is fast, the decomposition of the metal organic acid salt including TFA salt is insufficient. Consequently, there is a tendency that the solvent or the organic acid salt remains in the film of the oxide superconductor precursor which is obtained by the preliminary calcination. Therefore, at the time of the temperature rise of the subsequent heat treatment for crystallization, the organic acid salt of the remaining such as fluoride decomposes rapidly, and such as the bumping evidence, the extraneous material, or pores are generated in the film. Besides, the stress occurs in the film by the volume shrinkage at the time of the decomposition of the preliminary calcination film and the formation of the crystal of YBCO (shown as Y-type (123) superconductor), and the cracks which originate on such as the bumping evidence, the extraneous material, or pores occur.
By repeating the coating and the preliminary calcination heat-treatment, this tendency becomes remarkable when the film of the oxide superconductor precursor of multilayer structure is formed and when the thick film is formed. Consequently, because the cracks remain as it is when crystallizing the obtained precursor thick film and when obtaining the superconducting film, the Jc characteristic deteriorates remarkably by disturbing the current pathway at the time of flowing the electric current.
For solving these problems, by controlling the rate of temperature rise in the preliminary calcination heat-treatment and by making the metal organic acid salt decompose sufficiently, the process to accomplish the high Jc and the formation of the thick film is known (for example, refer to Patent document No. 1).
Besides, by controlling the preliminary calcination heat-treatment temperature at the time of the heat treatment of the oxide superconductor precursor which is formed onto the substrate, and/or, by controlling the water vapor partial pressure of introduced gas in the atmosphere of the crystallization heat-treatment, the process for producing the tape-shaped oxide superconductor of the thick-film which has the high orientation and the high Jc is known (for example, refer to Patent document No. 2).
However, in the above-mentioned process which controls the rate of temperature rise in the preliminary calcination heat-treatment, or, in the above-mentioned process which controls the preliminary calcination heat-treatment, and/or, in the above-mentioned process which controls the water vapor partial pressure of introduced gas in the atmosphere of the crystallization heat-treatment, although the production of the thick film was achieved than before, the film thickness was limited to approximately 1.0 μm. And the cracks occurred when the film thickness reached approximately 1.5 μm even in the improved process of the crystallization heat-treatment, and therefore, it was difficult to obtain the thick film which had the high Jc and the high Ic.
From the subsequent study, the applicants which compose this application have knowledge that the deterioration of the Jc which accompanies such production of the thick film or the Ic which is lower than the value which is forecasted originates not only the occurrence of the cracks but also the deterioration of the electric connectivity of crystal grain boundary. And, the applicants filed ever the process for producing the thick film tape-shaped RE-type (123) superconductor which had the high Jc and the high Ic by removing or restraining the cause of such occurrence of the cracks and the deterioration of the electric connectivity of the crystal grain boundary (Application No. 2006-226421).
In this process, after coating the raw material solution including metal elements which compose the RE-type (123) superconductor onto the substrate, the preliminary calcination is given, and subsequently, the RE-type (123) superconductor is produced by giving the heat treatment for forming the superconductor. When the molar ratio of RE, Ba and Cu in the above-mentioned raw material solution is RE:Ba:Cu=1:y:3, the segregation of Ba can be restrained by reducing the molar ratio of Ba at the range of y<2, for example, within the range of 1.0≦y≦1.8 (preferably 1.3≦y≦1.7). Consequently, the occurrence of cracks is restrained by restraining the precipitation of the impurities of Ba base in the crystal grain boundary And, the electric connectivity among the grain boundaries improves, and then, the tape-shaped RE-type (123) superconductor which has the uniform thick film and the superior superconducting characteristic is produced easily with high speed by forming the superconducting film by MOD process.
However, in the tape-shaped RE-type (123) superconducting wire which is produced by the above-described TFA-MOD process, the grain boundary characteristic and the crystalline property of the superconductor are improved by controlling the component of the solution, and the improvement of the Jc at the self field, that is, the improvement of the Jc at 77K, 0 T (tesla) is confirmed. Nevertheless, the Jc in 77K, 1 T receives the affect of the angular dependence for magnetic field. Because the Jc,min is low at 0.19 MA/cm2 and the angular dependence for magnetic field of the Jc shows the anisotropy at Jc,min/Jc,max=0.47, the introduction of the magnetic flux pinning centers is required into the superconductor for utilizing to the equipment which is used under the applied magnetic field.
As one process for solving this problem, the process which introduces the magnetic flux pinning centers into the superconductor by forming the Y0.77Sm0.23Ba1.5Cu3Oz superconductor that a part of Y is substituted to Sm by using TFA-MOD process onto the substrate was tried. According to this process, the particulate Sm-rich phase (Sm1+xBa2−xCu3Oz) which is the low-Tc phase is formed as the magnetic flux pinning centers into the superconductor, the angular dependence for magnetic field of Jc at 77K, 1 T is improved, and the anisotropy is improved at about 1.3 times with Jc,min/Jc,max=0.6. However, because the sizes of the magnetic flux pinning centers are large, the angular dependence for magnetic field of Jc is large as before.
Besides, according to S. V. Ghalsaki, etc., when forming Y0.33Sm0.66Ba2Cu3Oz superconductor that a part of Y is substituted to Sm by using TFA-MOD process onto the LaAlO3 single-crystalline substrate, the process which adds BaZrO3 particles is reported (for example, refer to Non-patent document No. 1).
According to this process, although BaZrO3 particles are added for forming the magnetic flux pinning centers, the film thickness is thin at about 0.2 μm, and Zr compound which forms the magnetic flux pinning centers is large at more than 30 nm, besides, the dispersion state is nonuniform. Therefore, the problem of the anisotropy is not solved.
On the other hand, according to J. GUTIERREZ, etc., when forming the YBCO superconductor onto SrTiO3 single-crystalline substrate by using TFA-MOD process, the process which adds BaZrO3 salt is reported (for example, refer to Non-patent document No. 2).
According to this process, although BaZrO3 salt is added for forming the magnetic flux pinning centers, the film thickness is also thin at about 0.2 μm, and Zr compound (BaZrO3) which forms the magnetic flux pinning centers is large at more than 5-dozens nm, the dispersion state is nonuniform, besides Zr compound disperses at the neighborhood of the substrate intensively. Therefore, because the angular dependence for magnetic field of Jc at 77K, 1 T stays at Jc,min/Jc,max=0.66, also the problem of the anisotropy is not solved.    Patent document No. 1: Japanese Patent Publication No. 2003-300726    Patent document No. 2: Japanese Patent Publication No. 2003-34527    Non-patent document No. 1: IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, No. 2, JUNE 2007    Non-patent document No. 2: nature materials/VOL 6/MAY 2007